Car batteries are prone to self discharge and also since there are onboard electronics that work even when the car is stopped (maybe 70 milli-amps), batteries that are left in cars that are not used will discharge after long periods.
The typical consequence of this is that after prolonged periods of non-use, a car will not start. In an extreme case the uncharged battery will sulfate irremediably. Recycling dead batteries also wastes energy and is detrimental to the environment. Though some damaged batteries will still start cars, typically, these require changing after a short period of use. This last case is quite costly for car manufacturers whose cars may not be started several months between the time they leave a manufacturing plant and a time they are purchased by a consumer.
It is not practical to have automotive employees starting cars, periodically, though even this does not completely solve the problem since some car batteries have to be run a long time and at high revolution to charge properly.
Disconnecting the battery only partially addresses the problem since it does not prevent self-discharge. Additionally, it brings new problems as some electronics will reset themselves and will need reprogramming. Further, it is very likely that the battery will later be reconnected which involves some labor.
Some prior art systems teach connecting a solar panel across the battery that was sized so as to compensate for the energy loss. This improves the situation but has a major shortcoming. Sunlight and battery drain conditions vary dramatically so in a number of cases the panel overcharges the battery. Overcharging of batteries is also destructive. If the panel is made small to not overcharge some batteries, then it may not properly charge others when subject to different conditions.